Canon, in the context of a fictional universe, comprises those novels, stories, films, etc., that are considered to be genuine or officially sanctioned, and those events, characters, settings, etc., that are considered to have existence within the fictional universe. In order for a setting to appear cohesive, especially in fictions that contain multiple parts, both creators and audiences sometimes find it useful to define what has and has not "actually happened" in that universe.
Material that is considered canon usually comes from the original source or author of the fictional universe, while (some) spin-offs and adaptations to other media are more likely to be non-canon and fan fiction is almost always non-canon.
While David Eddings never issued a clear canon policy of his own, there are various contradictions between the main series books of The Belgariad and The Malloreon and the two "autobiographies" Belgarath the Sorcerer and Polgara the Sorceress. These differences might be explained by rationalizing that the later books are in character and based on those character's memories, but this is soley fan speculation.